When a transmitted packet is not normally received at a receiving end in a wireless communication system, a method for retransmitting the corresponding packet is used. A hybrid automatic repeat request (HARQ) is one example of a retransmission method. In the HARQ, a bit error is primarily prevented through forward error correction, and the retransmission of a packet is requested through an automatic repeat request (ARQ) when an error occurs.
The HARQ may be classified into a chase combining HARQ (CC-HARQ) and an incremental redundancy HARQ (IR-HARQ) according to a method for combining retransmitting packets. In the CC-HARQ, a transmitting end retransmits the same packet and a receiving end performs a soft-combining of received packets in a symbol unit or a bit unit and then decodes them. In the IR-HARQ, the transmitting end retransmits redundancy bits that have been punctured in the previous transmission, and the receiving end connects the packet received in the previous transmission and the redundancy bits received in the retransmission and decodes them.
Meanwhile, among modulation methods, a quadrature amplitude modulation (QAM) is a modulation method that converts a plurality of bits of transmission data into phase and amplitude information of one symbol and transmits them. 4-QAM can transmit 2 bits in one symbol, 16-QAM can transmit 4 bits, and 64-QAM can transmit 6 bits in one symbol.
The 2n-QAM divides the transmission data by n bit unit, maps them to one of 2n symbols, and modulates and transmits them. At this time, gray mapping is mainly used as a symbol mapping method. When receiving the n bit symbol modulated according to the gray mapping, a difference in reliability of each bit of the received symbol occurs, and in particular, a large difference in reliability occurs in a situation in which a signal-to-noise ratio (SNR) is low. The reliability of the received bits may be represented as an average log likelihood ratio (LLR).
Therefore, a retransmission method for solving the difference in reliability is required.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.